Decorative and wear-resistance thermoplastic resin sheets have been used widely to provide protective coverings for floors, ceilings, walls, upholstery, automotive interiors, etc. As a result of popular demand, these thermoplastic sheets have often been provided with surface textural effects to simulate tiles, wood grain, bricks, and similar surfaces. These effects have been obtained by embossing, using mechanical or chemical techniques.
Mechanical embossing generally involves engraving a roll or plate, or otherwise treating its surface to create a desired design in raised relief. Then, either a thermoplastic sheet to be embossed or the embossing itself, or both, are heated and the embossing design is pressed into the softened thermoplastic resin sheet. Mechanical embossing in this manner has a number of disadvantages. Probably the greatest of these lies in the large capital expense and the difficulty of providing a uniform pressure along the length of an embossing roll or over the entire surface of an embossing plate. Therefore, this technique has generally has been limited to producing narrower thermoplastic resin sheets of up to twelve feet, the widest sheet commercially available. However, even in the mechanical embossing of narrower sheets, registery of the embossed design with the printed design has been a major problem requiring constant adjustment and resulting in considerable defective or out-of-registry products.
Chemical embossing techniques have become generally known since the issuance of U.S. Pat. Nos. 3,293,094 and 3,093,108 to Nairn, Harkins, Ehrenfeld and Tarlow on Dec. 20, 1966. The Nairn et al, references described a process in which a thermoplastic resin sheet is chemically embossed. The resin layer contains a chemical blowing agent, and the decomposition temperature of the blowing agent is altered by the selective printing of an inhibitor on its surface. The printed sheet is heated to a specific temperature range to decompose the blowing agent where no inhibitor is present. This results in differential foaming of the thermoplastic resin, producing depressed areas and raised areas. Since the inhibitor may easily be added to a pigment ink, the embossed effect can easily be registered in accordance with the printed pattern of the inhibitor and ink composition.
Another technique of chemical embossing is described in U.S. Pat. Nos. 3,464,934, 3,819,783, and 4,244,899. These references disclose a process in which the decomposition temperature of a chemical blowing agent is controlled by selectively applying an ink containing an activator to the surface of a foaming thermoplastic resin sheet. In this technique, subsequent heating is controlled at a lower temperature, so that areas of the foamable thermoplastic resin sheet in contact with the activator ink will foam to provide raised areas in accordance with the printed pattern of the activator and ink composition.
In these processes, a single foamable thermoplastic resin layer is provided. The bubbles in the foamed resin layer would tend to obscure any design lying beneath the foamed layer. If a second layer of embossing with different colors and designs is desired on top of the first layer, the second layer of foamable thermoplastic will obscure the design of the first layer when it is foamed. Therefore, multi-layer embossing has not been successfully achieved except by mechanical embossing or using a transfer sheet provided with a foamable plastisol, as described in Japanese Published Patent Application No. 47,065 of 1980. Mechanical embossing continues to suffer from the disadvantages described above, however, and embossing with transfer sheets involves increased process steps and expenses. Chemical embossing with a foamable plastisol is possible. But, because of the time required for the gelling of such plastisols, conventional high speed printing cannot be used. For example, if rotogravure printing with a plastisol were to be employed, the cell depth must be increased to 100 to 200 microns, and only 20-40 dots could be printed per linear inch since the thickness of the printed plastisol needs to be in the range of 0.02 to 0.06 mm. Moreover, the plastisol layer must be given time to gel before further processing can take place. Therefore, the speed of printing must be decreased, probably to as low as 30 feet per minute, to obtain acceptable results. Since conventional rotogravure printing is presently performed at speeds approaching 200 feet per minute, the slow speed of rotogravure printing with plastisols on a gelled base is impractical and unacceptable.
It is, therefore, an object of this invention to provide a printable composition which can be printed by conventional rotogravure apparatus and which will impart an embossed effect to thermoplastic resin sheets.
It is another objective to provide a process in which multi-layered chemical embossing is possible using conventional apparatus and process steps in order to avoid large capital expenses.
It is a further objective of the present invention to eliminate the need for mechanical embossing so that thermoplastic resin sheets can be provided in widths of up to fifteen feet or wider.
It is a further objective to provide thermoplastic resin sheets with multi-layered embossing effects.